Integrated Circuits (IC) surround us today and are present in almost every device, apparatus or accessory. Generally IC's consist of a variety of electronic components constructed as a single piece of semiconductor, built up in layers, manufactured in a process called “photolithography”.
In photolithography a semiconductor wafer (usually made from silicon) is first coated with a photoresist (light-sensitive) layer. Then, the coated wafer is exposed to a predetermined pattern of light that is acquired by passing light (laser or lamp light) through a reticle (sometimes called “mask”)—a blank, usually made form fused silica (quartz) or silicon dioxide, having an engraved pattern on it. Lastly, the irradiated wafer undergoes chemical development washing off the wafer to remove the exposed (or unexposed) coating, depending on positive or negative photoresist type, and etching that cuts through the wafer's uncoated areas.
Phase shift masks (PSM) and optical proximity corrections (OPC) are two major ways to overcome the restrictions imposed by diffraction, in photolithography in the semiconductors industry.
Photolithography exposure tools, such as steppers and scanners, were improved dramatically over the past decade, for smaller and smaller feature size on integrated circuits chips.
New generations of smaller and more densely packed chips, had required the advance towards higher numerical aperture lenses, shorter wavelength of light sources, better process control methods, and eventually, the use of some resolution enhancement techniques (RET).
PSM techniques, appeared as necessary for smaller design rules, for steppers exposure tools using I-line Hg lamp sources (wavelength 365 nanometer (nm)) and later, for the “deep-U.V.” Excimer laser sources—KrF (248 nm) and ArF (193 nm).
Current methods of PSM manufacturing, require additional lithographic and etching processes, to make actual grooves in the Quartz plates, which in turn, add dramatically to process variations, lower reliability, cost, and high sensitivity to aberrations of the steppers/scanners optics.
Lithographic process requires the use of costly Electron-beam (E.B.) equipment, which prints sequentially the lines onto the coated photosensitive material (“Resist”) on mask.
Following the coating and E.B. exposure, more costly processes are necessary, such as Resist development; Resist stripping, Etching and inspection.
To-date the manufacturing process of the reticle itself is generally made in two methods: one method is electron-beam manufacturing—a rather expensive and slow (but accurate) method—and the other method is laser patterning.
The latter method is very similar to the process described hereinabove with respect to IC's. A blank of fused silica (or silicon dioxide) is coated on one surface with a layer of chrome and a photoresist coating. An additional protective layer of anti-reflective layer (to prevent reflection inflicted defects on the reticle) is placed on top of the chrome layer. Laser irradiation is irradiated on the coated surface in a predetermined pattern, producing a latent pattern that later is developed in a chemical process, whereby the irradiated (in positive) or non-irradiated (in negative) is removed. Then the exposed portions of the chrome layer are removed in etching, and finally the remaining photoresist layer is removed (washing).
It is evident that the laser patterning process for the manufacture of reticles involves some five steps—rendering this process lengthy and susceptible to defects. Indeed this precarious process of reticle manufacturing is characterized by low cost-efficiency and high rate of quality-control rejections. Typically reticles may cost some 6,000 to 8,000 US Dollars apiece, and as high as some 50,000 US Dollars for a phase shift mask (with shift phase corrections). It is noted that laser patterning utilizes continuous wave (CW) laser light.
Typically the semiconductor industry employs light-sources producing deep UV grade irradiation, either in the 248 nanometer wavelength (0.248 micron), or in the more recently introduced 193 nm wavelength, for the scanner (stepper) in the photolithography process.
Direct writing on a reticle blank using CW laser appears to be inadequate. It should be noted that the quality of reticles has to be at least equal or even greater than the desired quality acquired in the production of the end product (the integrated circuit wafer). Conventional CW lasers fail to provide the resolution required by the industry (generally between 1.5 to 0.5 micron) as thermal effects creates collateral damage.
In recent years a new line of lasers was introduced—ultrafast pulsed lasers. These lasers generate ultra-short pulses (typically in the order of 10−13 seconds). The introduction of ultrafast pulsed lasers has brought about the ability to produce high power pulses, thus facilitating tasks that CW lasers were unable to accomplish as it required power beyond their limits.
The present invention aims at providing novel method and apparatus for manufacturing reticles.
An object of the present invention is to provide novel method and apparatus for manufacturing reticles that is substantially cheaper yet highly accurate and high-yielded.
Another object of the present invention is to provide novel method and apparatus for manufacturing reticles that has fewer stages than the known laser patterning method.
Further object of the present invention is to harness the advantages of ultrafast pulsed lasers to facilitate higher accuracy and high-yield in the manufacturing of reticles.
Yet another object of the present invention is to provide novel method and apparatus for inhibiting diffraction effects in reticles.
Other objectives and advantages of the present invention will become clear and apparent after reading the present specification and viewing the accompanying figures.